Fusible core materials for producing a hollow plastic part must be strong, flowable, and light. The development work of synthetic plastic core has so far been unsuccessful because of the difficulty of decoring,i.e., the plastic core material does not melt away easily and cleanly. Generally the plastic materials are light but they flow under a shear force due to the high viscosity of polymer phase. The present invention intends to overcome the preceding decoring problem by generating a shear force via magnetic force such that a clean melt-out without any residual polymer materials on the inside wall surface can be achieved.